1. Field of the Invention
The present invention relates to an interpolation method for a binary image, and more particularly, to an improved interpolation method in which a variable threshold value used for determining a pixel value to be generated by interpolation is determined according to a context (the state value of adjacent pixels). This invention has been adopted in ISO/IEC JTC1/SC29/WG11 N1903 (ISO/IEC 14496-2 Subpart 7 Committee Draft).
2. Description of the Related Art
Recently, a function for processing shape information has been added to MPEG-4. The shape information, which refers to object information of the image, is expressed as a binary image. To code such a binary image, MPEG-4 adopts a context-based arithmetic encoder (CAE). For lossy shape coding a down sampling method and an up sampling method are performed in each macro block of binary shape. A shape image is divided into shape blocks having an M.times.M size.
The down sampling refers to a method for reducing the binary image block according to a given conversion ratio. The reduced image block is transmitted together with the conversion ratio. Here, the conversion ratio is determined such that an error between the original binary image block and an binary image block restored later is within a predetermined range.
The reduced image block obtained by the down sampling is coded by a CAE and then transmitted.
The up sampling method is used to restore such a reduced image block. Up sampling is for restoring the reduced image block into block with the size of the original binary image block, by interpolation.
In such an up sampling process, an effective interpolation method must not cause excessive blocking and smoothing effects in the restored binary image.